Polyimide resins that contain pyromellitic anhydrides [1,2,4,5-benzene tetracarboxylic dianhydrides] as the main component have only limited solubility in a majority of solvents. As a consequence, a two-step method of preparation is currently in use. In that method, nearly equal mols of acid dianhydride and aromatic diamine are added in a polar solvent such as N-methylpyrrolidone ("NMP") or dimethylformamide and so on and reacted at room temperature or a lower temperature to obtain a high molecular weight polyamic acid solution. The polyamic acid solution is then cast on a substrate sheet. The sheet is then chemically treated or heated, to form a polyimide.
A one-step polymerization reaction by direct imidization of acid dianhydride and aromatic diamine would greatly simplify preparation of the polyimide, and help bridge the gap between a laboratory preparation method and a commercial manufacturing process.
It is known that certain kinds of polyimide 0 resins, those that contain biphenyl tetracarboxylic dianhydride or benzophenone tetracarboxylic dianhydride as the main component, can be made by a one-step polycondensation method which does not go through the high molecular weight polyamic acid. These methods depend on the resin's solubility in a specific solvent system.
Japanese Kokai Patent SHO 50-113597 (1975) describes a method for preparation of solvent-soluble type polyimides by reacting nearly equal mols of a diamine component and a tetracarboxylic acid component containing mainly biphenyl-3,4,3',4'-tetracarboxylic acids or 2,3,3',4'-biphenyl tetracarboxylic acids in a para-chlorophenol solvent system.
In Japanese Kokai Patent SHO 64-5603 (1989) and Japanese Patent SHO 61-45652 (1986), nearly equal mols of tetracarboxylic dianhydride and aromatic diamine are used and heated in para-chlorophenol, to obtain a polyimide resin composition which is dissolved in the halogenated phenol by one-step polycondensation reaction. And, in Japanese Kokoku Patent SHO 64-14994 (1989), 2,5-di(4-aminophenyl)-3,4-diphenyl thiophene is used as the aromatic diamine to react with benzophenone tetracarboxylic dianhydride, to obtain a polyimide resin composition which is soluble in m-cresol. In Japanese Kokai Patent SHO 54-10338 (1979), 4,4'-diamino-dicyclohexyl methane and tetracarboxylic dianhydride are heated in m-cresol at 50.degree.-160.degree. C., to obtain a polyimide resin composition directly. In Japanese Kokai Patent SHO 58-79018 (1983),dimethyl benzhydrol-3,3',4,4'-tetracarboxylate as the acid component and bis(4-amino-phenyl) methane are heated at 170.degree. C. in m-cresol, to obtain a polyimide resin that is dissolved in m-cresol.
Because the halogenated phenols being used in the above examples have potent toxicity, it is necessary to have a well-prepared working environment to prevent its dissipation. Therefore, there is a demand for a solvent or solvents that have lower toxicity than halogenated phenols and can dissolve the polyimides. The drawback of using solvents such as m-cresol, xylenol, and so on, is that these solvents cannot dissolve the polyimides of the present invention.